Multi-cavity molding, in particular for injection molding, is widely used for manufacturing multiple parts in a single mold during each cycle. Manifolds are required to direct molten material, e.g. plastics and metals, to a number of outlet ports through flow channels so that the molten material can be directed to multiple cavities to form parts. Hot runner manifolds use electric heating elements to keep the temperature of flow channels at the melting temperature of the molten material. During a production run, the heated material remains molten in the flow channels between shots, hence reducing material waste in the runner system and reducing the amount of finishing work required for the final parts.
Hot runner manifolds are widely used and may be the only economic way to manufacture multiple small parts in one shot. As the hot runner manifolds require embedded electric heating elements to keep the material molten in flow channels, it is critical to ensure that a sound seal is obtained around the flow channels to reduce or eliminate leaking of molten material to the heating elements or the outside of the manifold. In addition, the manifold design preferably allows the heating elements to be placed in such a way that an even temperature gradient across the mold can be achieved.
U.S. Pat. No. 5,496,168 issued Mar. 5, 1996 to Renwick describes a hot runner manifold in which matching grooves are machined in opposing surfaces of two steel plates to form flow channels as well as separated heating element channels. Heating elements and their channels are furnace brazed together, while the matching surfaces of the two steel plates are brazed to form an integrated manifold. This design is complex requiring an inordinate amount of machining to produce the filling ducts/recesses and air ducts leading to heating element channels, and the brazing ducts leading to one matching surface. Further, considerable amount of work is require to tack-weld individual filling tubes in a recess over each filling duct, fill braze powder into the ducts/tubes, and machine out filling tubes after furnace brazing. Furthermore, expensive vacuum furnace equipment is require to braze the heating elements and braze the two halves of the manifold into an integrated unit. Still further, brazing requires heating the entire manifold to about 1925° F., which deteriorates the mechanical properties of the manifold material and may cause deformation of the entire manifold. Yet further, the entire procedure to produce the integrated manifold requires a longer production cycle. Finally, furnace brazing may induce excess brazing material to leak into the flow channel, which will require expensive polishing procedures to clean.
U.S. Pat. No. 4,648,546 issued Mar. 10, 1987 to Gellert describes a composite plate method of manufacturing an integrated hot runner manifold. A manifold having two-halves is machined with matching grooves for flow channels, and another channel is machined on the upper external surface for the heating elements. Furnace brazing is used to seal the heating elements with the channels and to seal the matching surfaces of the two halves of the manifold.
U.S. Pat. No. 4,761,343 issued Aug. 2, 1988 to Gellert describes a manifold system having a bridging composite plate manifold interconnecting a number of support composite plate manifolds with different flow passage orientations to improve streamlined and uniform flow and reduce pressure drop while allowing flexibility of system design for different applications.
U.S. Pat. No. 5,227,179 issued Jul. 13, 1993 to Benenati describes a manifold assembly having interlocking components to contain the high pressure generated in the injection molding presses. A duct structure is designed to provide passages for the heated plastic. The duct contains a tubular member for flow channel, which is embraced by a two-half interlocking conduit cover. Four heating elements are embedded and sealed below the external surfaces at the four corners of the two halves to heat the flow channel. Manufacturing of the individual duct and interlock elements is very labor intensive.
U.S. Pat. No. 6,749,422 issued Jun. 15, 2004 to Yu describes a hot runner manifold having two separable halves with matching grooves to form flow channels. Molten plastic flows through ground channel pipes within the grooves. The flow channels are heated by heaters located in grooves on the external surface of manifold halves. The pipes are covered by copper plates, which act to improve heat transfer from the heaters to the flow channels to keep the plastic in a molten state. Manufacturing interconnected channel pipes is complex and expensive.
In U.S. Pat. Nos. 4,648,546, 4,761,343, 5,227,179 and 6,749,422, all flow channels are located at the matching surfaces of two halves of the manifold, while heating element channels are located on external surfaces. While reducing the risk of plastic leaking into the heater element channels, these designs are costly to manufacture, undesirably large and/or inefficient at heating the flow channels.
U.S. Pat. No. 6,099,292 issued Aug. 8, 2000 to McGrevy describes a hot runner manifold having a single block with flow channels machined therein. A serpentine groove is machined into the surface of the block to accept a heat conductive assembly, the heat conductive assembly being a conduit having a heater element therein. The serpentine groove and heat conductive assembly essentially parallels the path of the flow channels in the block. In this design, the manifold is a single block and the heater element is on the outside surface of the block. While reducing the risk of plastic leaking into the heater element channels, this design is costly to manufacture and inefficient at heating the flow channels.
German Patent Publication 10243387 published Mar. 18, 2004 to Holger describes a hot runner manifold system in which a first hot runner system may be connected to a second hot runner system. Each hot runner system may be a single block with grooves therein or two halves with grooves between them. Electric heater elements are used to heat the flow channels. There is no provision for sealing the flow channels away from the heater elements.
Japanese Patent Publications 5200786 and 5200787 both published Aug. 10, 1993 to Katsutoshi et al, describe a hot runner manifold having two halves in which leakage of resin from the runner is prevented by means of a core inserted into recessed grooves that surround the hot runner in both halves of the manifold. A recession in the core collects resin that leaks between the two halves and the pressure of the resin forces the core to tightly fit into the recessed grooves thereby forming a seal to prevent further leakage of the resin. Such a sealing method is not efficient and prone to failure. Furthermore, this design permits resin to pool in the recession and permits resin pooled in the recession to bleed back into the hot runner channel contaminating subsequent shots of resin. Thus, this design limits the ability to change resin type or color without disassembling the entire manifold.
Despite advances that have been made in the art, there remains a need for improved manifolds for distributing molten material to mold cavities, in particular improved hot runner manifolds in which leakage of molten material from the runners is reduced.